Help with XML::Twig xpath syntax, please

I have a simple XML file which contains details of files in a directory (the format is in the sample program below). I want to use XML::Twig (which is in widespread use elsewhere in the program suite) to pull out document elements corresponding to certain criteria, but I cannot for the life of me work out the syntax for the get_xpath method, despite browsing Michel Rodriguez's XMLTwig site and his xpath tester page.

Here's a sample program; what it's intended to do is to pull out the "bkfile" element which has dbid=392; it does that, but pulls out much else besides - including the other bkfile element whose dbid doesn't match - and I can't find out what I'm doing wrong. All help very gratefully received. XML::Twig is a great module and I've used it extensively elsewhere; it would be a nuisance to have to use some other XML module for this task.

This is XML::Twig 3.20 running under ActiveState Perl v5.8.7 and Windows XP (fully patched).

-------------- sample program -------------
use strict;
use warnings;

use XML::Twig;

my $xmldoc = new XML::Twig;
my $xmlsource =<<"ENDXML";
<bkdirectory>
  <bkfile>
    <dbid>393</dbid>
    <b_size>177</b_size>
  </bkfile>
  <bkfile>
    <dbid>392</dbid>
    <b_size>460</b_size>
  </bkfile>
</bkdirectory>
ENDXML

$xmldoc->parse($xmlsource);

my @bkfiles = $xmldoc->get_xpath('//bkfile[string(dbid)="392"]]');

print "\@bkfiles has ",scalar @bkfiles," elements\n\n";
foreach my $elt (@bkfiles) {
	print "----",$elt->name,"\n";
	$elt->print;
	print "\n";
}
-- output from sample: one line may fold, sorry -------
F:\>tryit.pl
@bkfiles has 11 elements

----bkdirectory
<bkdirectory><bkfile><dbid>393</dbid><b_size>177</b_size></bkfile><bkfile><dbid>392</dbid><b_size>460</b_size></bkfile></bkdirectory>
----bkfile
<bkfile><dbid>393</dbid><b_size>177</b_size></bkfile>
----dbid
<dbid>393</dbid>
----#PCDATA
393
----b_size
<b_size>177</b_size>
----#PCDATA
177
----bkfile
<bkfile><dbid>392</dbid><b_size>460</b_size></bkfile>
----dbid
<dbid>392</dbid>
----#PCDATA
392
----b_size
<b_size>460</b_size>
----#PCDATA
460

--

Henry Law       <><     Manchester, England
.